It is well known that it is very difficult to form an ohmic contact to a p-type zinc selenide layer. This is particularly so, even for a heavily doped P-type zinc selenide.
The cause of this problem is probably due to the relatively high Schottky barriers of the various metals used to make the contact, these barriers being about 1eV.
There have been many attempts to solve this problem. Thus, Uemoto J. Appl. No. 622-297359 teaches forming an ohmic contact by growing a p-type amorphous Si layer by a photo-assisted CVD method on the p-type ZnSe layer and then depositing Mo and Al on the Si layer. However, since there is no grading of the p-type amorphous layer, a barrier of only a few tenths of a volt may result in the prevention of the formation of an ohmic contact. Additionally, since there is no continuity of the lattice of the ZnSe layer into the deposited amorphous Si layer, the amorphous layer probably can be easily peeled away from the ZnSe layer thus resulting in poor mechanical integrity of the Si-pZnSe contact.
Okawa J. Appl. No. 62 16921 teaches the formation of an ohmic contact by the evaporation of a gold layer directly on the surface of a p-type ZnSe layer to which nitrogen ion acceptors have been added. It has been found, however that the contact actually formed is not an ohmic contact but is a Schottky contact with a barrier of about 1.4 eV. Such a contact requires the application of about 15 V or larger to achieve current density in the order of 10 A/cm.sup.2.
Basol U.S. Pat. No. 4,456,630 teaches a method of forming ohmic contacts on 11B-V1A p-type doped layers by etching the surface of the layers with an acidic solution to form a surface that is non-metal rich, treating the etched surface with a basic solution and then depositing a conductive metal layer. The only example shown of the p-type material is p-type cadmium telluride. Moreover, the method shown here does not work for p-ZnSe.
McCaldin et al U.S. Pat. No. 4,123,295 teaches forming an ohmic contact on a p-type ZnSe layer by use of an intermediate layer of HgSe or HgS. However, in the absence of any grading of the HgSe layer (which is not shown) an ohmic contact is actually not formed but a Schottky barrier of about 0.7 eV is found.
Kamata J. Appl. No. 63-10826 teaches the formation of an ohmic contact on a p-doped ZnSSe layer by providing an intermediate layer of p-type GaAs between the metal contact layer and the ZnSSe layer. However, it has been found that here too an ohmic contact is actually not formed since the barrier between the p-type ZnSe and p-type GaAs is about 1 eV.